The source of good and evil has been the subject of debate of theologians and philosophers for time immemorial. However, Judaism's position on this issue has been crystal clear, that it is G-d that is the source of all good and evil:
"I (G-d) form the light, and create darkness; I make peace, and create evil; I am the LORD, that doeth all these things" (Is. 45:7)
Furthermore, G-d makes it very clear that His commandments, what you refer to as the Law, are the objective expression of that good and evil (Dt. 30:8-20) and that the commandments, along with fear of G-d, are the sum composite of the value of human existence (Ec. 12:13-14). This position is very consistent with the philosophies of Nietzsche and Sartre who say explicitly that without the existence of G-d that there is no objective basis for morality.
The covenant that the Jews have with G-d is eternal and cannot be overturned. Each successive covenant, first Noah, then Avraham, Yitzhak, and Yaakov. G-d says multiple times that the Commandments are binding throughout the generations. The commandment of the Sabbath is called an eternal covenant (Ex. 31:16-17) and the Torah says that the commandment of tzitzit (fringes) will be observed throughout the generations (Nm. 15:38). Other references to the eternity of the Jewish covenant can be found in Deuteronomy 29:28, Psalms 111:7-8, and elsewhere. Rabbi Tovia Singer has an excellent article debunking the idea that the New Covenant mentioned in Jeremiah 31:31-34 overturns or dismisses G-d's older Covenant that He established with the Jews at Mt. Sinai.
Beyond the idea that the Bible is clear that the covenant between G-d and Israel will never be overturned, the insight that the existentialist philosophers give us about the nature of right and wrong also make the idea that G-d would overturn his covenant with the Jews nonsensical. If we accept that an objective concept of good and evil can only exist if there is a G-d, as Isaiah states, it stands to reason that G-d would let the word know exactly what constitutes good and evil, especially if He plans on judging human behavior for the purposes of reward and punishment. The Revelation at Mt. Sinai completes the picture: a Creator, a created world, an objective morality, and a detailed breakdown of what exactly constitutes right and wrong. For G-d to overturn the Covenant and institute a whole new system, especially if not done in the same grand public fashion as the Revelation at Mt. Sinai (around two million people!), would make G-d a trickster god. If G-d is in fact a trickster god, who's to say that He hasn't changed the rules 1,000 times since the Torah was given? If G-d can promise an eternal covenant and then just change it, we are left with the same existential doubts as if G-d didn't exist at all.
I think it is safe to say that the Law as given to the Jews is perfectly fine to follow, which is why Jews have continued to follow it for the past 3,300 years.